One form of magnetic separation device which functions by magnetic particle entrapment is generally referred to as a High Gradient Magnetic Separator or HGMS. An HGMS comprises a canister containing a liquid permeable packing of magnetizable material between the canister inlet and outlet. The packing material may be paramagnetic or ferromagnetic and may be in particulate or filamentary form, for example, it may comprise wire wool, wire mesh, knitted mesh or steel balls. The packing may be in the form of a single block which essentially fills the canister or it may be other forms, for example, concentric cylinders or rectangular plates. The term "matrix" is generally employed to refer to the packing and this is used, in the case where the packing is divided into a number of elements, by some in the industry to refer to the individual elements and by others to refer to the totality of the packing. The term will be employed herein in the latter way.
The canister is surrounded by a magnet which serves to magnetize the matrix contained therein, the magnet generally being arranged to provide a magnetic field in the direction of the canister axis. With the matrix magnetized, a slurry of fine mineral ore or clay in water is fed into the inlet of the canister. As the slurry passes through the canister the magnetizable particles in the slurry are magnetized and captured on the matrix. Eventually, the matrix becomes substantially completely filled with magnetizable particles and the rate of capture decreases so that the quantity of magnetizable particles in the treated slurry leaving the outlet of the canister reaches an unacceptably high level. The slurry feed is then stopped and the canister rinsed with water to remove all non-magnetic material from the matrix. The magnetic field is reduced to zero and the matrix is scoured with high-speed wash water to remove the magnetizable particles therefrom.
The processing capacity of an HGMS is proportional to the product of the surface areas of the matrix to which the slurry is fed and the velocity of the slurry through the matrix. It is also dependent on the depth of the matrix since the greater this is, the more chance that a magnetizable particle will be trapped. However increasing the length beyond the limit required to ensure satisfactory performance, that is, to give a reasonable chance of particle capture, does not enhance the capacity of the separator. An increase in slurry velocity will increase capacity but will also cause a corresponding decrease in the possibility of capture of a magnetizable particle. Thus, the velocity can only be increased to a certain limit since, beyond this, the quality of product will suffer.
Accordingly, efforts to increase capacity have been centred on designing HGMS with large matrix surface areas. This has led to the employment of a single element matrix of large diameter with a relatively short axial length located within a correspondingly shaped magnet. With electromagnets, pole pieces are arranged at each end of the canister around the inlet and outlet thereto to concentrate the flow of magnetic flux longitudinally through the matrix. The limiting factor on the size of matrix elements which can be used for such arrangement is the maximum depth of the magnetic field which it is possible to achieve.
A problem with the arrangement described above is that it cannot be employed efficiently with the process described in U.S. Pat. No. 4,124,503. In that process, two canisters are provided, alternatively movable into a magnetised zone. While one of the canister is in this zone, the other is being rinsed and washed. This process is very economical and practical since it allows almost continuous treatment of feed slurry, the feed being stopped only when the canisters are actually being moved. For best results with an HGMS, a super-conducting magnet is employed and this is indicated to be preferred for the process of U.S. Pat. No. 4,124,503. It is the use of a super-conducting magnet and the need for canisters which can be moved into and out of a magnetic field which prevents a short, large diameter matrix being employed efficiently in the process of U.S. Pat. No. 4,124,503. There are two reasons why this is so as follows.
Firstly, a uniform field is required for good results and this can only be obtained with the short, large diameter magnet, which is necessary to use with a short, large diameter matrix, by employing iron pole pieces. However, the use of such iron pole pieces means that the canisters cannot be readily moved into and out of the magnetic coil which is an essential feature of the operation of the process of U.S. Pat. No. 4,124,503. Secondly, super-conducting magnetic design favours a coil whose length is about twice its diameter, this arrangement providing a laterally uniform magnetic field without the need for pole pieces. This form of super-conducting magnetic provides a higher field than is achievable with a shorter magnet with iron pole pieces since, in the latter case, a limit is set by the fact that iron saturates at a magnetic flux equivalent to approximately 2 Tesla whereas in the former case a uniform field is readily achievable with magnetic fluxes equivalent to about 5 Tesla. However, of course, this form of super-conducting magnet dictates that the matrix is also of a length twice its diameter, i.e., the complete opposite to the desired matrix aspect ratio discussed above.
In order to increase effective surface area, within the constraints provided by the use of super-conductive magnet and the need for the canisters to be removable from the magnetic, U.S. Pat. No. 4,124,503 proposes employing a matrix in the form of a tube, the slurry being fed into the centre of the tube and then radially outwards therethrough. Other suggestions for maximising the matrix surface area are, for example, to provide multiple thin cross-section matrix elements arranged parallel to the axis of the canister in the form of two rectangular sections, a series of concentric tubes or as an array of rectangular sections. However, all these arrangements suffer from the deficiency that the flow of the slurry through the matrix is transverse to the axis of the canister and hence to the direction of the magnetic field. It is known that the effectiveness of the capture of magnetizable particles is less when the slurry is fed through the matrix transverse to the magnetic field than when it is fed parallel thereto.
GB 1388779 describes a separator with plural matrix elements stacked in a chamber and feed means for feeding fluid through the elements in a direction parallel to the magnetic field within the chamber. The feed means comprises a separate supply pipe for each matrix element which feeds a flow control member positioned above the element. Each flow control member includes a distribution network having a central chamber and plural radial passages for feeding the element therebelow and a collection network of similar form for receipt of slurry from the element thereabove. The arrangement is relatively complicated and vulnerable to failure by blockage of the radial passages.
It is an object of the invention to provide a magnetic separator which is simple in form but achieves equal if not better results than known separators.